The present invention relates to an image forming device which utilizes, for example, an electro photographic method and electrostatic recording method to repeatedly form a large number of images; and, more specifically, the present invention relates to an image forming device that has a control unit which performs adjustment of the toner density and bias voltage in a developing unit according to a value detected by a patch sensor representing the image density of a recorded image and a value of current flowing in an image carrying body from the developing unit.
In a conventional recording device using an electro photographic method, a photoconductive photo-sensitive body is uniformly charged, an image is exposed thereon, a partial discharge of electric charges is produced caused on the photosensitive body and an electrostatic latent image is formed thereon. Thereafter, the electrostatic latent image is developed with toners containing coloring agents. As an example of developing units which supply toners for the electrostatic latent image formed on the photosensitive body, other than a developing unit of the one component developing type, which uses only toners for the developing agent, there is a developing unit of the two component developing type, which uses a developing agent containing toners and carriers. Finally the toner image thus obtained is transferred and fixed on a recording medium, such as a recording paper, to obtain a record image. These processes are repeatedly performed.
The two component type developing, which uses a developing agent containing both toners and carriers, will be explained. When performing a development by supplying toners to a photosensitive body, since the toner density in the developing agent gradually decreases with the formation of successive images over time, it is required to maintain the toner density in the developing agent; therefore, when the toner density decreases to some extent, a toner replenishing the supply with unit is driven for replenishing new toners. For this reason, a toner density control unit is required, which always keeps the toner density constant.
A conventional toner density control unit will be described. The toner density in the developing agent containing both toners and carriers is measured with a toner density detector provided in the developing unit. When the output value of the detector falls below a reference set value, it is judged that the toner density has decreased, and the toner replenishing unit is driven until the value received from the toner density detector reaches the reference set value, so as to replenish the toner supply with new toners. The reference set value is determined in advance for every predetermined number of printing pages, so as to supplement the characteristic change of the developing agent, thus the toner density is always controlled to be constant.
The toner density control corresponding to a record image will be explained. The image density of a patch image transferred on a recording medium, such as a recording paper, during the transfer process is read by a patch sensor, for example. The read value is fed back to a control unit, such as a CPU in the machine main body, and, when the read image density does not reach a reference value, the toners are replenished until the reference value is reached. Thereby, through such toner density control, the image density of the record image is always kept constant, as disclosed in JP-A-2002-23436, for example.
In the conventional toner density control used in connection with two component type development, the toner density is controlled for every predetermined number of printing page; and, if a problem occurs during the interval, the image density of a patch image is directly detected and the detected value is fed back to the control system to control the toner density.
Thereby, even midway of the predetermined printing pages, the printing quality is finely controlled. However, in the conventional toner density control, there is a problem in that, when deterioration of the image density of a patch image is caused due to an abnormal property change of the developing agent, and the deterioration is countermeasured by modifying the developing conditions, such as the toner density and bias voltage, the property of the developing agent is further deteriorated and the image density of the record image can not be kept constant.